Pfthb

I do user and invitation validation using the Optional facility

@DeleteMapping("/friends/username")
public
HttpEntity<Boolean> removeFriend(
 @ApiParam(value = "The user's name", required = true) @PathVariable String username
) {
 Long fromId = authorizationService.getUserId();

 return userService.findByUsername(username)
 .map(user -> 
 return friendshipService.findFriendship(fromId, user.getId())
 .map(friendship -> 
 friendshipService.removeFriendship(friendship);

 friendship.setToId(friendship.getFromId());
 friendship.setFromId(friendship.getToId());

 friendshipService.removeFriendship(friendship);

 return ResponseEntity.ok(true);
 ).orElseGet(() -> ResponseEntity.notFound().build());
 ).orElseThrow(() -> new ResourceNotFoundException("User not found"));

However, IntelliJ is colouring my grey return
https://zapodaj.net/2f48b1a26c392.png.html
But when I remove the return, it highlights to me that there is no return https://zapodaj.net/37605f08165c9.png.html

Could someone explain how it works and what is it all about?

Your statement lambda

param -> return expression; 

can be changed to an expression lambda:

param -> expression

Simple, isn't it? Note, that the curly brackets and the semicolon need to be removed.

Sometimes I found useful to leave the
braces where they are if the block of code is long enough (I think it improves readability)

In Android Studio you can locally disable the warning using //noinspection CodeBlock2Expr at the start of the method like in the example below

//noinspection CodeBlock2Expr
button.setOnClickListener((View v) -> 
 //a long single method call...
);